The goal of this application is to understand the interactions of von Willebrand factor (Vwf) with platelet membrane glycoprotein Ib/IX and with subendothelial collagen. In this First Aim, three dimensional structures of recombinant Vwf A1 domain with and with a GpIb peptide will be determined by x- ray diffraction. This will be done with collaboration with Dr. Liddington. In the Second Aim, conformational changes in the Vwf A1 domain that allow Vwf to interact with platelet GPIb will be identified by developing conformationally dependent antibodies. In the Third Aim, the x-ray structure of the Vwf A3 domain will be determined and used as a guide for mutagenesis studies that characterize the interaction of Vwf A3 with collagen. In the Fourth Aim, potential interactions between the A1 and A3 domains will be defined by direct binding studies of the individual domains as well as by obtaining an X-ray structure of the A1A2A3 Vwf polypeptide. In the Fifth Aim, a novel form of von Willebrand's disease characterized by defective collagen binding will be defined.